Stroke, silent cerebral infarct (SCI), and cognitive impairment are frequent and highly morbid complications of sickle cell disease (SCD) in children. Current approaches to the prevention and treatment of neurological complications of SCD include screening by transcranial Doppler ultrasound (TCD) to identify children with elevated cerebral blood flow velocity who are at increased risk for strokes; these children are then typically treated with chronic transfusions indefinitely. Hydroxyurea (HU) reduces the frequency of painful crisis, acute chest syndrome and transfusion and may have beneficial effects on central nervous system (CNS) complications of SCD. The safety of HU in infants and children has been demonstrated recently in a NIH-sponsored phase III trial; however, the exact indications for the use of HU in children remain unclear, as well as its efficacy in preventing CNS complications of SCD. Our preliminary data suggest that, if the cumulative frequency of abnormal TCD, SCI and stroke could be reduced by 50%, the majority of pediatric hematologists would prescribe HU to all young children with SCD. The long term goal of this project is to perform a primary prevention trial to demonstrate the neuroprotective effect of HU and broaden the indications for HU in children. The goals of this proposal are to: 1) conduct a feasibility trial demonstrating the acceptability of a randomized trial of HU to reduce the CNS complications of SCD; 2) demonstrate that sedation for MRIs can be safely performed in young children with SCD using a standardized protocol; and 3) create the leadership, network of clinical centers and other procedures necessary to conduct a definitive phase III trial demonstrating the efficacy of HU for primary prevention of neurological complications of SCD. The primary endpoint for the feasibility and definitive phase III trials will be the development of abnormal TCD, SCI or stroke. To begin the feasibility trial, we have obtained CTSA support for pilot studies at Johns Hopkins and Washington University; over the next two years, these sites will screen 40 participants 12-48 months of age and randomly assign and follow 20 participants for two years. Two additional centers (Children's Hospital of Philadelphia and the University of Alabama, Birmingham) will begin enrollment during the course of the R34 (20 patients screened and 10 participants randomly assigned per site), to provide a total of 80 participants screened, 40 randomly assigned, and a minimum of 70 participant years of follow-up. Participants must have TCD measurements that are well below the threshold for transfusion and MRIs that are without evidence of SCI. Participants in the pilot studies will continue into the proposed R34 and phase III trials, to complete 3 years on HU or placebo. The information from the feasibility trial is necessary to demonstrate the safety and practicality of a definitive phase III trial. The results of these studies could lead to true primary prevention of CNS complications of SCD, including abnormal TCD, SCI, neurocognitive impairment and stroke. In doing so, this study could also reduce the burden of chronic transfusions and change clinical practice by broadening the indications for HU.